1. Technical Field
The present invention relates in general to the field of electronics and lighting, and more specifically, to a system and method for controlling and/or providing power to current regulated light sources, such as light emitting diode light sources.
2. Description of the Related Art
Commercially practical incandescent light bulbs have been available for over 100 years. Recently, other light sources have begun to show promise as commercially viable alternatives to the incandescent light bulb. For example, light emitting diodes (LEDs) are becoming particularly attractive as light sources in part because of long life, energy savings through high efficiency light output, and environmental incentives such as the reduction of use of mercury.
LEDs are semiconductor devices and are driven by direct current. The brightness of an LED varies in direct proportion to the current flowing through the LED. Thus, increasing current supplied to an LED increases the brightness of the LED, and decreasing current supplied to the LED dims the LED.
FIG. 1 illustrates a representative prior art LED lighting system 100, which includes a switching power converter 102. Voltage source 101 supplies an alternating current (AC) input voltage Vin(t) to a full bridge rectifier 103. Voltage source 101 can be, for example, a public utility, and the AC voltage Vin(t) is, for example, a 60 Hz/110 V line voltage in the United States of America or a 50 Hz/230 V line voltage in Europe. Full bridge rectifier 103 rectifies the input voltage Vin(t) and supplies a rectified, time-varying, line input voltage Vx(t) to switching power converter 102.
Switching power converter 102 includes a switch 108 that operates in response to a control signal CS to regulate the transfer of energy from the rectified, time-varying input voltage Vx(t), through inductor 110 to capacitor 106. Switching power converter 102 additionally includes a diode 111 that prevents reverse current flow from capacitor 106 into inductor 110.
Energy transferred through inductor 110 is stored by capacitor 106. Capacitor 106 has sufficient capacitance to maintain an approximately constant voltage VC while providing current to a load, such as LED lighting subsystem 112. In at least some implementations, switching power converter 102 is a boost-type converter in which voltage VC is greater than the peak of input voltage Vx(t).
In operation, input current iin varies over time, with a peak input current proportionate to the “on-time” of switch 108 and with the energy transferred to capacitor 106 proportionate to the “on-time” squared. Thus, this energy transfer process is one example of a nonlinear process. In some implementations, switch 108 is an n-channel field effect transistor (FET), and control signal CS is a pulse width modulated (PWM) control signal that causes switch 108 to conduct when the pulse width of CS is high. Thus, in such implementations, the “on-time” of switch 108 is determined by the pulse width of control signal CS, and the energy transferred from Vx(t) to capacitor 106 is proportionate to a square of the pulse width of control signal CS.
Power control system 100 also includes a switch state controller 114 that generates control signal CS with a goal of causing switching power converter 102 to transfer a desired amount of energy to capacitor 106, and thus, to LED lighting subsystem 112. The desired amount of energy depends upon the voltage and current requirements of LED lighting subsystem 112. To provide power factor correction close to one, switch state controller 114 generally seeks to control input current so that input current iin tracks input voltage Vx(t) while holding capacitor voltage VC substantially constant.
In practical systems, however, link voltage VL is subject to ripple, which can cause visible flicker in the LEDs of the LED lighting subsystem 112. Such flicker will become even more severe if LED lighting system 100 includes a phase-cut dimmer to dim the LEDs of LED lighting subsystem 112. The present application recognizes that it would be desirable to efficiently operate an LED lighting subsystem off of line voltage with reduced link voltage-induced ripple and flicker.